Aves
Birds, also known as Aves or avian dinosaurs, are a group of ic s, characterised by s, less ed jaws, the of eggs, a high rate, a four-chambered , and a strong yet lightweight . Birds live worldwide and range in size from the to the . They rank as the world's most numerically-successful of s, with approximately ten thousand living species, more than half of these being s, sometimes known as perching birds. Some birds, especially and s, are among the most s; several bird species , and many pass on knowledge across generations, which is considered a form of . Birds produce offspring by laying s which are fertilised through . They are usually laid in a and by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as , lay eggs even when not fertilised, though unfertilised eggs do not produce offspring. Birds have wings which are more or less developed depending on the species; the only known groups without wings are the and s. Wings, which evolved from s, gave birds the ability to , although further evolution has led to the loss of flight in s, including s, s, and diverse . The and s of birds are also uniquely adapted for flight. *The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth. Most birds are highly adapted for rapid digestion which reduces weight to aid with flight. **Birds short digestive systems make them ideal for seeds. Ingestion of seeds that are capable of resisting digestive juices allows the seeds to be scattered in feces and . For these seeds, gut passage enhances germination ability. *Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. The record demonstrates that birds are modern feathered , having evolved from earlier s within the group, which are traditionally placed within the n dinosaurs. The closest living relatives of birds are the ns. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group , have been found dating back to the mid- period, around 170 million years ago. (The first flowering plants are known from 160 mya.) Many of these early "stem-birds", such as , retained primitive characteristics such as teeth and long bony tails. DNA-based evidence finds that birds diversified dramatically around the time of the 66 million years ago, which killed off the s and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving s according to . Phylogenetic tree |1= (ostriches) |label2= |2= (rheas) |label2= |2= |1= ( & s) |2= (kiwi) |2=† (elephant birds) }} }} }} |2= (tinamous) |2=† (moa) }} }} }} }} |label2= |2= |1= ( s and relatives) |2= ( s and relatives) }} |label2= |2= |1= ( s) |2= (oilbird) |2= (potoos) }} |2= (frogmouths) |2= ( s and s) }} }} }} }} |label2= |2= |1= |1= (turacos) |label2= |2= (bustards) |2= (cuckoos) }} }} |label2= |2= (pigeons) |label2= |2= (mesites) |2= (sandgrouse) }} }} }} }} |label2= |2= ( and ) |label2= |2= |1= |1= (flamingos) |2= (grebes) }} |2= ( s and relatives) }} |label2= |2= |1= (tropicbirds) |2= ( and ) }} |label2= |2= ( s) |2= |1= ( es and s) |2= (penguins) }} }} |2= (storks) |2= ( , s, etc.) |2= ( s, s & es) }} }} }} }} }} }} |label2= |2= (hoatzin) |label2= |2= |1= (New World vultures) |2= ( s and relatives) }} }} |2= |1= (owls) |label2= |2= (mouse birds) |label2= |2= (cuckoo roller) |label2= |2= (trogons and quetzals) |label2= |2= ( s and relatives) |label2= |2= ( s and relatives) |2= ( s and relatives) }} }} }} }} }} }} |label2= |2= (seriemas) |label2= |2= (falcons) |label2= |2= (parrots) |2= (passerines) }} }} }} }} }} }} }} }} }} }} }} }} }} References Category:Tree of life